High-precision time transfer technology finds valuable and important application in the fields of satellite navigation, aerospace industry, deep space exploration, geodesy, and tests of fundamental physics. Current satellite-based time transfer technologies, such as GPS common view (CV) and two-way satellite time transfer (TWSTFT), have a level of precision at the nanosecond. As clock sources with high stability and low uncertainty, such as optical lattice clock, are invented and applied, the current technologies no longer meet the needs of scientific research and social development. A technology of time transfer by laser link (T2L2) with a theoretical uncertainty of better than 100 ps is currently still in development. Furthermore, although the space-based time transfer technology has been proved to be quite mature and feasible, it has disadvantages such as complex system, high cost, long averaging time, poor security, low reliability, and so on. Fiber-optic transmission has the advantages of low loss, large capacity, high speed, high stability, safety, and reliability, and has been widely used in the field of communications. Fiber-optic based time transfer is an effective way to realize high-precision long-distance time transfer. High-precision fiber-optic time transfer is faced with the problem that the transmission delay of the fiber link changes with changes in temperature, stress, transmission wavelength, and other factors. To realize high-precision time transfer, bidirectional time transfer method is widely adopted at present. For long-distance bidirectional fiber-optic time transfer, bidirectional optical amplification must be performed to compensate the attenuation of optical signal. AGH university of Poland discloses a single fiber bidirectional optical amplifier. See Śliwczyński et al., “Dissemination of time and RF frequency via a stabilized fiber optic link over a distance of 420 km,” Metrologia, vol. 50, pp. 133-145 (2013 January). Bidirectional amplification of single fiber is realized by replacing isolators at either end of the unidirectional EDFA with filter, and the bidirectional propagation delay symmetry is ensured. However, serious noise accumulation caused by multiple optical amplifications, such as Rayleigh backscattering around optical amplifiers, seriously deteriorates the signal-to-noise ratio and limit the total length of the fiber link. See Śliwczyński et al., “Bidirectional Optical Amplification in Long-Distance Two-Way Fiber-Optic Time and Frequency Transfer Systems,” Instrumentation and Measurement, vol. 62, pp. 253-262 (2013).
National Metrology Institute of Japan (NMIJ), see M. Amemiya et al., “Precise frequency comparison system using bidirectional optical amplifiers,” Instrumentation and Measurement, IEEE Transactions on, vol. 59, pp. 631-640 (2010), discloses a bidirectional optical amplification scheme for wavelength division multiplexing (WDM) fiber-optic time transfer scheme. Using the WDM to separate and combine wavelengths from two directions, isolators inserted can suppress multiple amplifications of noises such as Rayleigh backscattering. Nevertheless, the improvement of signal-to-noise ratio in the scheme is at the expense of the bidirectional transmission delay asymmetry. The asymmetry has to be calibrated and the total calibrating error increases linearly with the increase of the number of amplifiers.
The CESNET in Czech, see Vojtěch et al., “All optical two-way time transfer in strongly heterogeneous networks,” Proc. of SPIE, pp. 92020S-92020S-6 (2014), discloses distributed Raman amplifier to carry out optical amplification of time signal for bidirectional transmission. Although the bidirectional symmetry of the time delay of fiber-optic time transfer link can be ensured, the noise such as Rayleigh scattering can also be multiple optically amplified. Moreover, there are other problems such as high pump power, low efficiency, dependence on the polarization of the input optical signal, and so on.
The two bidirectional optical amplification schemes as disclosed in Chinese Patent Application numbers CN201610216342.6 and CN201610073321.3 can ensure the symmetry of the link to the maximum extent, and effectively avoid the influence of multiple amplifications of noise such as Rayleigh scattering on the time transfer performance of the fiber-optic.
For distributed fiber-optic time transfer, AGH university of Poland, see P. Krehlik, et al., “Multipoint dissemination of RF frequency in fiber optic link with stabilized propagation delay,” IEEE transactions on ultrasonics, ferroelectrics, and frequency control, vol. 60, pp. 1804-1810 (2013), discloses inserting a 2×2 optical coupler in the main link, resulting in a portion of forward and backward transferred optical signal for distributed time transfer. However, the approach reduces the power of the optical signal transferred by the main link and also degrade the time transfer stability of the main link.